The present invention relates to a pretensioner for straining a belt of a seatbelt system when an emergency occurs. More particularly, the present invention relates to a clutch mechanism used for such a pretensioner.
Vehicles, for example, automobiles, are provided with a seatbelt system which is designed to restrain the occupant's body to the seat. It has been proposed to attach a pretensioner to the seatbelt system as a means for sensing excessive acceleration that acts on the vehicle when an emergency situation, e.g., a collision, occurs, and for straining the belt which has been loosely fastened to the occupant's body, thereby enhancing the restraining effect. Pretensioners are disposed at various parts of the seatbelt system and generally arranged to wind the belt tight, to pull down the buckle, to pull the intermediate portion of the belt, etc., on the basis of the operation of a power source with an acceleration sensor used as a sensing means. Among them, the pretensioner that is designed to wind the belt tight is generally disposed in association with the retractor.
Incidentally, when it is in an inoperative state, the pretensioner that is attached to the retractor must be mechanically disengaged from the belt take-up shaft so as not to prevent the belt unwinding and rewinding operation of the retractor. Accordingly, a clutch mechanism is interposed between the driving element of the pretensioner and the belt take-up shaft.
A conventional clutch mechanism of this type has a driving rotary member and a driven rotary member, which are disengaged from each other. When the pretensioner is activated, an engagement member is wedged into a wedge-shaped space defined between the driving and driven rotary members by using a phase difference in the direction of rotation of these rotary members, thereby connecting the two rotary members. A typical example of this type of clutch mechanism is disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 58-195571 (see the specification of U.S. Pat. No. 4,558,832).
However, in the conventional clutch mechanism having the above-described arrangement, the operation of wedging the engagement member into the wedge-shaped space mainly depends on the sliding frictional resistance occurring between the engagement member and the driven rotary member. It is not always easy to ensure the frictional resistance and secure the reliability of clutch engagement.
On the other hand, the conventional arrangement necessitates the engagement member to be temporarily locked to a stationary element, that is, the pretensioner housing or the like. Therefore, when the pretensioner is activated, the engagement member may be brought into an unstable position by the operation of canceling the temporarily lock, e.g., cutting, which may obstruct a smooth wedging action.